cx.background_executor().spawn(...) -> cx.background_spawn(...) (#25103)

Done automatically with

> ast-grep -p '$A.background_executor().spawn($B)' -r
'$A.background_spawn($B)' --update-all --globs "\!crates/gpui"

Followed by:

* `cargo fmt`
* Unexpected need to remove some trailing whitespace.
* Manually adding imports of `gpui::{AppContext as _}` which provides
`background_spawn`
* Added `AppContext as _` to existing use of `AppContext`

Release Notes:

- N/A

crates/command_palette/src/command_palette.rs

@@ -281,7 +281,7 @@ impl PickerDelegate for CommandPaletteDelegate {
             query = alias.to_string();
         }
         let (mut tx, mut rx) = postage::dispatch::channel(1);
-        let task = cx.background_executor().spawn({
+        let task = cx.background_spawn({
             let mut commands = self.all_commands.clone();
             let hit_counts = cx.global::<HitCounts>().clone();
             let executor = cx.background_executor().clone();